Stabilization of polyphosphate fertilizer solutions

ABSTRACT

A minor excess of fluoride over that required to react with all the aluminum and magnesium in ammonium and potassium polyphosphate liquid fertilizer solutions will prevent the precipitation of these elements. Effectively, this addition of excess fluoride shifts the solution composition from a region where the water-insoluble precipitate MgAl(NH4)5(P2O7)2F2.6H2O (I) is stable to a region where the water-soluble form, dimorph II, is stable; thus the metallic cations are sequestered. Alternatively a smaller amount of fluoride is added and the resulting precipitate is filtered to obtain a stabilized solution.

United States Patent 1 Frazier [541 STABILIZATION OF POLYPI-IOSPHATEFERTILIZER SOLUTIONS 75 Inventor: Alva w. Frazier, Florence, Ala.

[73] Assignee: Tennessee Valley Authority 221 Filed: May 7, 1971 [21]Appl. No.: 141,366

Related US. Application Data [63] Continuation of Set. No. 82,809, Oct.21, 1970, and a continuation-in-part of Ser. No. 30,264, April 20, 1970,abandoned.

[52] US. Cl ..71/34, 71/64 C, 210/51, 210/57 [51] Int. Cl. ..C05b 7/00[58] Field of Search ..71/1, 33, 34, 64 C, DIG. 2,

[56] References Cited UNITED STATES PATENTS 3,290,140 12/1966 Young..71/34 3,044,851 7/1962 Young ....23/ l 07 3,024,099 3/ 1962 Martinson..71/34 3,088,819 5/1963 Funkhouser ..71/36 X 3,057,711 10/1962 Reusseret a1. ..71/43 3,554,728 1/1971 Moore et a1. ..7l/33 51 Jan. 16, 19733,585,021 6/1971 Geissler ..7 1/34 OTHER PUBLICATIONS 'The CondensedChemical Dictionary, 6th Edition,

l961-page 72 Frazier, Def. Pub. of Serial No. 82,809-filed 10/21/70,published in 890 0.6. 981 on 9/2l/7l-Def. Pub. No. T890,0l0 -71-34Primary Examiner-Reuben Friedman Assistant Examiner-Richard BarnesAttorney-Robert A. Petrusek [5 7] ABSTRACT 1 Claim, N0 DrawingsSTABILIZATION OF POLYIIIOSPHATE FERTILIZER SOLUTIONS This application isa continuation of my copending application Ser. No. 82,809, filed Oct.21, 1970, now Defensive Publication No. T890,0l which in turn is acontinuation-in-part of my copending application Ser. No. 30,264, filedApr. 20, 1970, now abandoned both for STABILIZATION OF POLYPI-IOSPHATEFER- TILIZER SOLUTIONS.

The invention herein described may be manufactured and used by or forthe Government for governmental purposes without the payment to me ofany royalty therefor.

My invention relates to a new and improved method for the stabilizationof ammonium polyphosphate liquid fertilizer solutions; particularly itrelates to the preven tion of precipitation principally of magnesium andaluminum salts in liquid ammonium polyphosphate fertilizer solutionsprepared from the ammoniation of acid of the wet-process type; and stillmore particularly it relates to the prevention of precipitation ofmagnesium and aluminum salts, i.e., the sequestration thereof by amethod of complexing ions of magnesium and aluminum salts present inammonium polyphosphate solutions from the ammoniation of wet-processphosphoric acid.

Liquid mixed fertilizers having compositions similar to those ofstandard dry-mixed fertilizers are well known in the industry and areever increasing in popularity. Such solutions have the advantage overdry mixed fertilizers in that costs of evaporating water and bagging areeliminated and, perhaps even more importantly, the application to soilis greatly simplified over the use of solid fertilizers and eliminatesdifficulty due to segregation and caking often encountered in storage ofdry fertilizers. However, liquid fertilizers have in the past had someoutstanding disadvantages. Raw-material costs have, up until the adventof wet-process phosphoric acid, been relatively high and the solutionsproduced, in many instances, have not been able to compete economicallywith solid fertilizer products. Solutions in the early development offertilizer solutions were limited to a maximum plant food content ofabout 33 percent by weight in that in concentration thereabove thereoccurred unwanted and undesired crystallization and precipitation ofsalts. One of several recent and perhaps the most significantbreakthroughs in overcoming these disadvantages in liquid mixedfertilizers is taught and described in U. S. Pat. No.

2,950,961, Striplin et al. Therein Striplin teaches that I he is able toprepare a liquid mixed fertilizer solution containing substantial valuesof the primary plant nutrients in a process wherein he ammoniates highpurity superphosphoric acid under controlled conditions. It is perhapsnoteworthy that although Striplins teachings appear to be mostlyconcerned with the ammoniation of superphosphoric acid of the furnacetype, he does not appear to be completely limited thereto in that hestates the source of his superphosphoric acid could be from any numberof means or processes including the evaporation of water fromorthophosphoric acid andpresumably could be of either the wet or furnacetype. Albeit this is taught in Striplin, there is claimed anotherbreakthrough in overcoming the disadvantages in liquid mixed fertilizersin application Ser. No. 835,377, Getsinger, assigned to the assignee ofthe present invention, and in U. S. Pat. No. 3,044,851, D. C. Young. Theimprovements disclosed therein generally relate to the production ofsuperphosphoric acid of the wet-process type by a separate heating andconcentrating step in which the starting acid is orthophosphoric. Theheat-treated acid is concentrated and condensed to bring about the insitu formation of pyrophosphoric, tripolyphosphoric, and higher speciesof polyphosphoric acid, such as to render the acid product similar inmany ways to what is now commonly referred to as furnace superphosphoricacid, the principal difference being in the amount or degree ofimpurities contained in the more impure wet super acid, which impuritiesare principally iron and aluminum, together with magnesium and othercongeneric impurities leached from the phosphate rock in the preparationof these acids.

In still another fairly recent breakthrough in overcoming some of thedisadvantages of producing liquid fertilizer solutions by the prior-artmethods, there is found in U. S. Pat. No. 3,382,059, Getsinger, theteaching of completely eliminating the separate heating andconcentration step for ortho-wet-process phosphoric acid by a directprocess which utilizes in a unique manner the heat of reaction fromammonia for directly producing ammonium polyphosphate solutions, melts,or suspensions from ortho wet-process phosphoric acid.

My invention is not directed primarily to the production of ammoniumpolyphosphate solutions principally derived from the ammoniation ofwet-process phosphoric acid, but rather it is directed to an improvementin stabilizing such ammonium polyphosphate solutions before or afterthey are produced, and my invention is directed specifically to suchsolutions which are derived, in whole or in part, from wet-processphosphoric acid. In the production of such solutions the grade of thefinished material is kept within the range from about 8240 to about11-37-0, and if all conditions for manufacturing and processing thereofare carefully controlled by now known prescribed methods, the resultingsolutions will sometimes remain, upon subsequent storage and handling,as true solutions withoutany discernible or significant precipitation ofsalts therein. However, when the finished grade of these solutionscontains greater than about 0.] percent MgO, for instance about anaverage value of 0.4 percent MgO (oftentimes resulting from the desiroususe of less expensive phosphate rock raw material), the industry hasthen experienced the unfortunate precipitation of principally magnesiumand aluminum salts in such liquid ammonium polyphosphate fertilizersolutions prepared from wet-process phosphoric acid. This problem is aserious one in the fertilizer industry and is encountered at all processand handling points between the manufacturer and the farmer applying thefertilizer in the field. The manufacturer encounters plugged valves,lines, and storage tanks; however, at this point, the required technicalabilities are usually available to circumvent the problem at hand.Still, however, drastic measures may be required at these points ofprocessing, as for example, excessive sediments in the storage tanks ofthe manufacturer can sometimes be removed only by means of employingexplosive action to dislodge the sediments thereby oftentimes weakeningthe storage tank structure. It is, however, after the product leaves themanufacturer that the lack of technically trained personnel complicatesthe precipitation problems several fold. For example, strong acidsolutions cannot be used to dissolve these precipitates which haveplugged river barges or railroad tank cars. Likewise, the sediment inthese instances usually arent deep enough to be susceptible to removalby explosives.

In my earlier work, I have helped to solve this specific problem ofprecipitates formed in ammonium polyphosphate solutions derived in wholeor in part from the ammoniation of wet-process phosphoric acid byshifting the solution compositions to a mildly acidic condition throughthe use of phosphoric acid until the solid phases representing theprecipitates are no longer stable and will dissolve slowly withoutdamage to the tank cars. However, these results of my earlier work haveproven to be time-consuming and require the acquisition and carefulhandling of phosphoric acid, which is usually not readily available ateither the distribution centers or to the farmers, who have the sameprecipitation problems in their equipment as do the manufacturers.

I have now developed a new, novel, and unique method for stabilizingsuch ammonium polyphosphate fertilizer solutions, near neutral pHconditions, derived in whole or in part from wet-process phosphoric acidand which are produced at concentrations or grades wherein theaforementioned precipitation problem occurs. These problems can now bealleviated through exploitation of my discovery that fluorine, as eitherfluoride or fluosilicate, is an efficient complexing agent for thecations in these solutions responsible for precipitate formation andthat this complexing agent will prevent the precipitation when added toor maintained at the proper concentration in such liquid fertilizersolutions.

In my work I have found that the most common precipitate in typicalammonium polyphosphate liquid fertilizers made from wet-process acidproducts of low fluoride content is a crystalline salt with the chemicalcomposition of MgAl(NH (P 0-,) -F -6I-I,0 (I) (waterinsoluble dimorph).In completely defluorinated wetprocess acid products, the compound thatprecipitates is Mg(Nl-l P 0 -4H 0 (orthorhombic dimorph). The amount offluoride remaining in wet-process acid products prepared in the usualmanner frequently is insufficient for precipitation of all the magnesiumas the complex fluoride compound, so that some Mg(NH.,) P 0 -4H 0 (0) isprecipitated also. Other precipitates containing magnesium or aluminumthat are known to occur in these fertilizer solutions, but are foundless often are Mg(NH P 0,'4I-I 0 (mono-clinic dimorph), Mg(NH p 0 '6l-l0, MgNH PO -c6l-I 0, and Al(NH cf20,OH-3H,0.

From these observations, therefore, I became aware that the problem ofsolid-phase formation associated with these liquid fertilizers centersaround the key impurities magnesium, aluminum, and fluoride. A seriouscomplicating factor that obscures their importance when these productsare initially produced is that these precipitation processes require anincubation period before troublesome crystallization begins. Thus,solidphase precipitation problems belatedly occur at unexpected pointsafter production and during handling, distribution, and fieldapplication. The typical conditions that arise due to delayedprecipitation are plugged valves, sludge buildup in storage tanks, riverbarges, or railroad tank cars, and formulation and handling problems atdistribution centers. Corrective measures are extremely difficult tocarry out at distribution centers or during field application becauseheretofore about the only effective method has been to adjust the pH ofthe solution to a low value, since acidic solutions are necessary todissolve these water-insoluble compounds.

This problem has been considered in the prior art and only two basicprocesses seem to have been proposed to decrease the severity of theproblem. In one process, the wet-process phosphoric acid is fullydefluorinated" Suzuki, Y., and l-iomma, l(., Removal of Fluorine fromPhosphoric Acid, Japan 6817416, July 23, 1968.) Shearon, G. B., andStevenson, G.L., Defluorination of Phosphoric Acid, US. Pat. No.3,429,663, Feb. 25, 1969.) by heating before ammoniation, thuspreventing the formation of MgAl(Nl-l 0 F -6H 0 (1) (water-insolubledimorph) which otherwise crystallizes readily as it has arelativelyshort nucleation period. The fluoride-free fertilizer liquid is stillcapable of forming unwanted precipitates, but the only mitigatingcircumstance here is that these precipitates usually require a longertime for crystallization to begin. Thus, if the products are producedand used within a relatively short time (2 to 6 weeks), the chance thata troublesome precipitate will form is small.

In the other process, the liquid product prepared from wet-process acidis diluted with about one-half its volume of the more expensive ammoniumpolyphosphate product prepared from high purity, electric-furnace gradesuperphosphoric acid, Striplin, M. M., Jr., Stinson, J. M. and.Wilbanks, J. A., US. Pat. No. 3,0l5,552, Jan. 2, 1962.) a material thatcontains no magnesium or aluminum impurities. This blending dilutes theimpurities contributed by the wetprocess acid to a lower degree ofsupersaturation so that the incubation period required for initiation ofprecipitation is lengthened. Here again, however, there is still the'risk of delayed precipitation, so that the product must be consumedwithin a limited storage time. This second process, however, is somewhatsuperior to the first because the dilution of any wetprocess acidproduct will decrease the total amount of possible precipitate byone-third. Both processes simply attempt to circumvent or decrease theseverity of this problem by lowering the content of magnesium, aluminum,or fluorine impurities. Another associated process which has beenaccepted to circumvent this problem is the production of suspensionfertilizer fluids.' Silverberg, .l., and Walters, H. I(., Com.Fertilizers, 108(4), 26-7, 66-7, April I964.) Slack, A. V., and Nason,M. C., J. Agr. Food Chem., 9 343-8 (Sept-Oct. 1961) Liquid Fertilizersfrom Wet- Process Phosphoric Acid. Suspension of Impurities) Slack, A.V., Farm Chemicals, 128 (5), 2l-2, 24, 26, May I965.) In this process,higher grade fertilizers are obtained by suspending solid phases whichare in equilibrium with the liquid phase in such a manner thatprecipitating impurities do not interfere with the characteristics ofthe fertilizers. For example, the impurities of magnesium, aluminum, andfluoride always precipitate in these products as very small crystalswhich are readily suspended when compared to the crystalline phases ofthe primary fertilizer materials, such as KCl, KNO (NH HPO.,, and/or NHH PO Again, however, this process has its inherent problems due toexcessive crystal growth of the primary nutrient materials to a sizewhich can no longer be suspended and also due to the breakdown of thesuspending agents which are available for this purpose.

It is therefore the principal object of the present invention tostabilize ammonium polyphosphate fertilizer solutions derived in wholeor in part from wet-process phosphoric acid and of grades up to about11-37-0 wherein the formation of undesirable precipitates isencountered, principally caused by the cations of magnesium and aluminumsalts by the controlled addition thereto of fluorine, as either fluorideor fluosilicate, as an efficient complexing agent for these ions tothereby render them unavailable for post-precipitation in the stabilizedsolutions.

1 have discovered that the foregoing and other objects of the presentinvention can be obtained by the addition of fluoride to these samesolutions in moderate amounts over and above the amount already presentto prevent the formation of these salts by shifting the solutioncomposition to a region in which only soluble complexes or theirwater-soluble salts are stable and can be maintained in solution,namely, the crystalline compounds MgAl(NH (P O,) F '6l-l O (Il)(water-soluble dimorph), Mg(NH (P O '6H O, and (NH.,) Al F6.

In carrying out the objects of my invention in the principal formsthereof, I have found that, for my process to be effective, the fluoridecontent must be held within a certain concentration range. If thefluoride exceeds the prescribed amount, precipitation of thesewater-soluble salts then becomes possible, thereby creating a problemsimilar to that experienced when the fluoride content is too low. On theother hand, additions of fluoride to a concentration below theprescribed level only increases the unwanted precipitation problem byforming more MgAl(l lH.,) (P O F -6H O (l) until one the requisiteconstituents (MgO, A1 0 or F) has been completely removed from solution.This discovery of an optimum range of fluoride or fluosilicate contentwhich gives maximum complexing properties offers a process by whichthese products can be essentially stabilized or clarified of magnesiumand/or aluminum precipitates that may otherwise separate. The amount tobe added has been established by experimental tests and may bedetermined by an empirical computation.

Several factors that control the fluoride sequestration of magnesium andaluminum by formation of soluble complexes have been tested in theseexperimental solutions. The most significant test results are summarizedin table I, infra. The samples used for these studies were prepared fromstock ammonium polyphosphate solutions along with other reagents to givedifferent concentrations of MgO, M 0 F, and polyphosphate and a range ofpH values. The mixtures were than seeded with the undesirable compound,MgAl(NH4)5201)2Fz'6H2O allowed to equilibrate at 25 C. at least 3 weeksbefore being filtered. The precipitates were examined microscopicallyand the filtrates were analyzed chemically. The compositions of theclear filtrates then were used to determine the extent to which thefluoride could sequester the magnesium and aluminum, thus giving aliquid fertilizer composition that would be stable or could precipitateonly negligible amounts of solids at this temperature. The tableincludes the conditions of each test, the composition of the liquidphase, and the stable solid phases found for these solutioncompositions.

A simple mathematical relationship has been devised to express theseresults in terms of a sequestration ratio (S. R.) which is herebydefined as a measurement to show the extent to which fluorine cancomplex magnesium, aluminum, and iron, if present, in a particularsolution composition. This value based on the data available for myoriginal application was previously expressed as S.R. wt.% A1 0 wt.%MgO/wt. F.

where the optimum value for SR. was between 1.3 and 1.6. Additionaldata, now included in table I infra, shows that much more fluoride isneeded to sequester aluminum than magnesium. Also, in order that the SR.value will be directly proportional to the quantity of requiredfluoride, the revised formulation is expressed as S.R. %F./wt. MgO 3 wt.A1 0 and the optimum value for S.R. is now 0.4 to 0.5 for productshaving 50 percent of the total P 0 as polyphosphate.

Since typical 10-34-0 or 1 1-370 fertilizer solutions have pH valuesclose to.6.0 and approximately 50 percent of their P 0 content aspolyphosphate, these conditions were used for most of the examples givenin table I, except when these variables themselves were being tested, asshown by the footnotes. In examining these results, it must beremembered that the magnesium and aluminum contents in thepost-precipitated, initial liquid products are lowered to as little as0.01 percent MgO and 0.03 percent A1 0 by the precipitation ofMgAl(NH.,) (P O F '6H O (I) so that higher concentrations representcations that have been sequestered. In the absence of fluoride, Mg(NH.,)P

O -4H O and Al(NH.,) P O,OH-3l-I O are the troublesome precipitates. Forammonium polyphosphate products prepared exclusively from wet-processacids that have low magnesium O.l percent) and fluoride O.1 percent)contents, the amount of precipitate is not a problem provided thealuminum concentration is no more than about 1 percent Al O However, thecurrent use of phosphoric acids reclaimed after use for picklingaluminum metal yields products with abnormally high aluminum contents.Excessive sludge formation results from the use of these acids with theresulting formation of Al(NH.,)- cf2O-,-OH-3H O, in amounts that make itimpossible to handle the product with conventional liquid-distributingequipment. Even in ammonium polyphosphate liquids such as those madefrom reclaimed acids, the addition of fluoride was effective inclarifying or redissolving the aluminum precipitate.

Storage temperatures also may influence precipitation, but preliminaryresults indicate that no further crystallization occurred when thesetreated solutions were stored at C. for 4 weeks. The solid phases thatwould be expected to form on cooling for longer storage periods arestill the water-soluble salts mentioned earlier; these would, of course,redissolve on warming to the original temperature. However, underconditions where solids may have formed at low temperatures and meansare not available for reheating these products, the solids could beremoved readily from clogged fertilizer equipment or storage tanks bysimply dissolving them in water. This type of cleanup process cannot beused to dissolve the water-insoluble salts which are the current problemin ammonium polyphosphate liquid fertilizers.

In order that those skilled in the art may better understand how thepresent invention can be practiced, the following examples of myimproved method for stabilizing ammonium polyphosphate solutions againstthe aforementioned undesirable precipitation are given by way ofillustration only and not by way of limitation, and more particularlythe following examples are of fered principally to show the extent ofthe complexing power of fluoride on magnesium, aluminum, and iron inammonium polyphosphate liquid fertilizers. The composition range forpercent MgO and percent M 0 in these tests were chosen to more thancover the values usually found in typical products, the averageconcentrations of which are about 0.5 percent MgO and 1.0 percent A1 0These values do not necessarily limit the compositions at which fluoridewill sequester these cations.

EXAMPLE I Effect of Ratio of Ortho-P O to Pym-P 0 The results shown byGroup A, Table 1 below, give the effect of the ratio of orthotopyrophosphate on the sequestration ratio. In my original application,this effect was masked by the failure of the S.R. formulation to includethe significance of aluminum. More reliable data at 70 percentpolyphosphate and an extension of this effect to percent polyphosphateshows that the polyphosphate level is a very significant factor indetermining the required quantity of fluoride, i.e., compare the SRvalues and polyphosphate levels for this first group of samples in tableI infra. These results show that polyphosphate units over and above thatrequired for sequestration of the iron and aluminum in liquid fertilizerare an uneconomical luxury which is magnifle d by the fact that anincreasing quantity offluoride is then required to sequester themagnesium.

TABLE I Sequestration by Fluorine of Impurities in AmmoniumPolyphosphate Fertilizer Solutions (Tests made at pH 6.0 and with equalamounts of orthoand pyrophosphate P 0 F added as NH F) +MgAIU) (I) 0.43

13" 0.70 0.40 0.25 34.0 None 0.12 8 '1 1.70 1.80 2.50 34.2 MgAlllI) 0.352H 1.45 1.50 2.10 35.6 MgAl (II) 0.35 3H 0.55 0.56 0.82 31.6 Mg! (11)0.37 C 2H 1.45 1.50 2.10 35.0 MgAl (II) 0.35 is 1.75 1.80 2.30 38.8 MgAl(11) 0.32 D 6513 1.50 1.00 2.90 35.0 MgN 0.64

565 1.60 0.45 3.60 35.4 (NI-I AIF MgN MgAl (II) 1.20 538 0.52 0.27 1.3037.3 (NHQ AIF MgN 1.00 598 0.21 0.04 1.20 35.6 (NH4)3A1F3 3.60 E 10A0.05 0.22 0.01 36.1 MgAl(1) 11d. 13A 0.03 0.03 0.15 39.4 MgAl (I) n.d.15A 0.13 0.44 0.01 37.8 MgAl(1) n.d. F I 0.16 0.50 32.2 0.90 Not idem.0.34 Plant 2 0.22 0.60, 0.77- 34.0 0.90 None 0.33 3 0.40 0.80 1.10 33.60.40 None 0.33 4 0.46 0.71 1.10 34.3 0.31 None 0.37 5 0.60 0.43 0.8533.7 0.90 v. minor (NI-LhAlF 0.39 6 0.27 0.45 0.63 35.0 0.70 V. minor(NH4)3AIF3 0.37 7 0.32 0.64 0.81 33.4 0.91 None 0.32

MgA g .)5( 1)= '6 z MgAl (11) S.R. wt. MgO 3 wt. M 0 15 wt. rep, insolution Ratio ortho to pyrophosphate P 0 70:30.

4 Ratio orthoto pyrophosphate P 0 30:70

" F added as (NH SiF n.d. not determined because of uncertainty of smallconcentrations. Commercial fertilizer solutions.

1 Ratio orthoto pyrophosphate P 0 90:10; Fe O 0.5 wt. percent.

EXAMPLE II Effect of pH Sample Group B, Table 1, supra, shows that thepH has an insignificant effect on the sequestration ratio. A slightlyhigher S.R. value is obtained at pH 6.5 as compared with that at pH 6.0,but it is still well below the recommended value 0.4 to 0.5 for percentpolyphosphate products.

EXAMPLE Ill Effect of Fluoride Source The pair of samples in Group C,Table 1, supra, show that fluorine as ammonium fluosilicate willsequester magnesium and aluminum as well as or better than fluorine asammonium fluoride. This effect is significant since the fluosilicate ioncan be obtained at low cost at fertilizer plants by scrubbing theoffgases from either electric-fumace or acidulation processes fortreatment of phosphate rock. Likewise, other soluble sources of fluoridehave been tested at a SR. value of 0.4 and have been found to be equallyeffective, for example, KF, K SiF KHF NH I-IF NaF, NaHF Na SiF HF, and HSiF EXAMPLE IV Effect of Excess Fluoride Group D, Table 1, supra, isgiven to show the effect of excess fluoride on the sequestration ratioand the solid phase composition. An increase in fluoride content gives alarger sequestration ratio for any composition and, at the excessivelevels shown by Group D, usually will result in the precipitation of oneor more of these solid phases. 1 found that increasing the fluoridecontent above that required for sequestration in commercial productswouldrapidly remove aluminum from solution as solid (NHQ AIF and moreslowly remove magnesium as Mg(NH (P O '6I-I O. These data are given toshow that, when concentrations are not known precisely or are determinedfrom compositions of the wet-process acid used for their production, itis better to have an excess of fluorine than a deficiency, since itwould ensure that any precipitated solids would be water soluble.

EXAMPLEV Effect of Insufficient fluoride The samples in Group E, TableI, were prepared initially to give sequestration ratios between 0.1 and0.3, but each mixture precipitated large amounts of MgA1(NH O,) F -61-IO (I). The chemical analyses of these mixtures show that, under theseconditions, the amount of the least of the three impurities, A1 MgO, andP, will determine the amount of precipitate. The precipitate formed atS.R. of 0.3 or below cannot be removed by washing with water. Based onthe'solid-phase composition, a sequestration ratio of 0.2 would effectthe most nearly complete precipitation of all three constituents fromsolution, so that removal of these impurities by filtration should yielda more desirable, clarified product.

EXAMPLE VI Test on Commercial Products For this example, seven freshcommercial 10-34-0 products prepared from wet-process phosphoric acidwere divided into several portions to which increasing amounts offluorine were added, the mixtures were seeded and allowed to equilibrate6 months with frequent agitation. Within 3 days at the insufficientfluoride levels, an increasing amount of MgAl(Nl'I (P O F -6H O (I) wasobtained with increasing fluoride content, showing that thisprecipitation is very rapid when seeds are present. When fluorineconcentrations were too high, the amount of solid (NlI,,) AlF increasedwith increasing fluorine content and the added seeds eroded anddissolved. Two samples of each series, which were in the optimum rangeof fluoride concentration remained clear or showed very minorprecipitation. The sample with the lower fluorine content sometimesyielded only a small amount of MgAl(NI-I O-,) F '6H O (I); the othersample deposited no precipitate and contained only the charged seeds;one mixture contained an insignificant amount of an unidentified solidphase. The chemical analyses of these fully sequestered samples areshown by. Group F. Since, in these mixtures, iron is the only othersignificant cation impurity that may be complexed by fluoride,consideration of a term for iron was necessary to maintain an S.R. of0.41 at a polyphosphate content of 50 percent; for example, suitablecorrelations could be obtained when one-third of the weight percent Fe Owas included in the equation.

These tests indicated that, for this type of fertilizer mixture, theamount of fluorine required to sequester iron is about one-ninth thatrequired for aluminum. It is to be recalled, however, that bothorthophosphate and pyrophosphate ions readily form complexes with iron--more so than with aluminum. On the other hand, aluminum forms verystable, soluble complexes with fluorine--more so than iron. Thus, itmust be concluded that the lower fluorine requirement of iron resultsfrom the existence of iron in complexes with phosphates.

Several other sludged commercial products have been clarified by theaddition of soluble fluorides. Many of these were three-componentfertilizer liquids in which KCl had also been included, specifically8-25-3 and 7-21-7 grades. The @254 had an abundant precipitate ofMgAl(NH .,(P O,) F -6H O (I) (water-insoluble form) and Al(NI-I.,) PO-,OH'3H O and required an addition of 3 percent fluoride forclarification to clear liquid. The 7-21-7 likewise contained MgAl(Nl-I(P O-,) F -6H O (I) and an equally abundant amount of an amorphousunknown gel phase which was misidentified as (Al,Fe)PO 'nl-I O in theoriginal application; 1.4 percent fluoride was required to produce aclear liquid fertilizer from this mixture.

EXAMPLE VII Effect of High Aluminum Contents Qualitative tests were madeon samples of commer cial 10-33-0 and 8-25-0, liquid fertilizersprepared from reclaimed phosphoric acid that had been used to picklealuminum. It was found that very small needle crystals of Al(NH.,) P OOl-I-3l-I O precipitated in these solutions and formed a badly gelledmixture that would not pour and could not be pumped from a railroad tankcar. The addition of ammonium fluoride equivalent to approximately 2percent F was made to this gel to provide an S.R. value of 0.4; within20 hours the precipitate was gone and the mixtures were very fluid. Infact, the treated material had higher clarity than most ammoniumpolyphosphate liquids prepared from wet-process phosphoric acid.

EXAMPLE VII This example represents new data obtained subsequent to thefiling of my original application due to the discovery thatpolyphosphate was an important factor. For this example, a 10-34-0liquid fertilizer was simulated from reagent chemicals at apolyphosphate level of 10 percent of the total P 0 with impurity levelsof 1.0 percent MgO, 1.0 percent A1 0 1.0 percent Fe O and 0.5 percent F.These cation impurities represent near maximum values when compared tocommercial l0-34-0 liquid fertilizers produced from wet-processphosphoric acids and the 0.5 percent fluoride represents the requiredS.R. value of 0.12 necessary to sequester the MgO. In preparing thissample, it was obvious that 10 percent polyphosphate was more thansufficient to sequester 1.0 percent Fe O which was charged as FePO, andrapidly dissolved, whereas the 1 percent A1 0 was charged as MP0, andrequired 2 weeks for dissolution showing that the 10 percentpolyphosphate is close to the required quantity for sequestering 1percent A1 0 This sample was chosen from a series of similar mixtureswhich included an increasing concentration for the impurities which alsoincluded 0.6 percent, 0.8 percent and 1.2 percent, the first two ofwhich rapidly became clear solutions and the last one not onlyprecipitated a significant quantity of MgAl(Nl-I (P=O,) F,-6H O but alsocontained undissolved AlPO 'nH O as charged. The

absence of a significant precipitate in this sample shows that thepyrophosphate content needs to be approximately five times the weightpercent of Fe O A1 Therefore, this discovery that pyrophosphate levelsabove this quantity for the purpose of sequestering iron and aluminumrepresents luxurious consumption of materials and also requires a higherfluoride level for sequestering magnesium, shows that the presentlyaccepted value by the fertilizer industry of 30 percent as a minimumpolyphosphate level is sufficient for approximately 6 percent Al O Fe Owhich is far above the average sum of l to 2 percent A1 0 Fe O andrepresents unnecessary expense.

While I have shown and described particular embodiments of my invention,modifications and variations thereof will occur to those skilled in theart. I wish it to be understood, therefore, that the appended claims areintended to cover such modifications and variations which are within thetrue scope and spirit of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A process for removing from ammonium polyphosphate fertilizersolutions containing about 50% of the phosphorous content aspolyphosphate, up to about 11 percent by weight nitrogen and 37 percentby weight phosphorus, expressed as P 0 said ammonium polyphosphatesolutions derived in whole or at least in part from the ammoniation ofwet-process phosphoric acid, the undesirable congeneric impuritiestherein principally magnesium and aluminum from said solution for thesubsequent removal by filtration of the resulting precipitation product,MgAl(NH F' O,) F .6H O(I), by adding to said ammonium polyphosphatesolution a soluble fluorine source selected from the group consisting ofNH F, NH HF, (NHg SiF KF, KHF K SiF NaF, NaHF Na SiF HF, H SiF andmixtures thereof, in amounts equivalent to a sequestration ratio ofbetween about 0.2 and 0.3, said sequestration ratio determined by theempirical formula Sequestration Ratio (S.R.) =Wt F/Wt.% MgO 3 wt.% A1 0if; wt.% Fe O and thereafter filtering the resulting precipitationproduct and thereby removing same from said ammonium polyphosphatefertilizer solutions.

